(1) Field of the Invention
The present invention relates to a weighing apparatus having a means for preventing interfering radio waves from entering an electronic device in the weighing apparatus.
(2) Description of the Prior Art
Combinatorial weighing apparatus of the prior art operate by supplying articles to be weighed to a plurality of weighing machines, combining weight values measured by the weighing machines into a number of weight combinations, comparing the weight combinations with a given weight, selecting a weight combination which is the same as or closest to the given weight, and discharging the articles from those weighing machines which give the selected weight combination.
Each of the weighing machines has a weighing hopper into which articles are charged and a load detector coupled to the weighing hopper. The load detector generally consists of a load cell having strain gages, and an amplifier including semiconductor devices such as diodes and a highly sensitive operational amplifier for amplifying a weak weight signal produced by the load cell.
Recently, an electronic weighing apparatus including a single scale plate and a display unit positioned adjacent to the scale plate for displaying the weight of an article of merchandise placed on the scale plate and also other items of information such as the name, the unit price, and the price of the article by using data items which are entered beforehand, have been used. The electronic weighing apparatus also has a load detector, which may be the same as the load detector in the combinatorial weighing apparatus, disposed below the scale plate.
Various radio waves in a wide frequency range are propagated in space. Such radio waves enter the amplifiers in the weighing machines through the weighing hoppers and scale plates. These weighing hoppers and scale plates act as an antenna, and the radio waves may be detected by a nonlinear portion in the amplifier and added to the weak weight signal thereby causing an error in the operation of the weighing apparatus. Other interfering radio waves such as sharp radio wave pulses generated by an automobile ignition system or the like also tend to interfere with the operation of the amplifier through the weighing hoppers and the scale plates, causing the weighing apparatus to malfunction.
To solve the above problem, Japanese Laid-Open Patent Publication No. 60-120222 discloses insulation interposed between a weighing hopper and a load cell for protection against radio interference. However, since the weighing hopper and the load cell are insulated from each other in a DC manner, a triboelectric charge developed by friction between an article being weighed and the weight hopper cannot be discharged to ground. The insulation between the weighing hopper and the load cell serves as a capacitor which allows a high-frequency signal induced by the weighing hopper to flow through the capacitor toward the amplifier, which may then fail to operate normally.